Comparative Studies on Porous Material-Supported Pd Catalysts for

Jul 2, 2009 - The feasibility and mechanism of five molecular sieves-supported palladium catalysts in elimination of benzene, toluene, and ethyl aceta...
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Ind. Eng. Chem. Res. 2009, 48, 6930–6936

Comparative Studies on Porous Material-Supported Pd Catalysts for Catalytic Oxidation of Benzene, Toluene, and Ethyl Acetate Chi He,† Jinjun Li,† Jie Cheng,† Landong Li,† Peng Li,† Zhengping Hao,*,† and Zhi Ping Xu*,‡ Department of EnVironmental Nano-materials, Research Center for Eco-EnVironmental Sciences, Chinese Academy of Sciences, Beijing 100085, People’s Republic of China, and Australian Research Council (ARC) Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology and School of Engineering, The UniVersity of Queensland, Brisbane QLD 4072, Australia

The feasibility and mechanism of five molecular sieves-supported palladium catalysts in elimination of benzene, toluene, and ethyl acetate were investigated. The experimental results indicate that Pd particles are well dispersed on these porous supports with size of 2-10 nm, while mesoporous supports with a larger specific surface area and pore diameter lead to a better dispersion. The activity of freshly made catalysts obeys the following order: Pd/Beta > Pd/ZSM-5 > Pd/SBA-15 > Pd/MCM-48 > Pd/MCM-41. Although Pd/Beta has the highest activity for removal of these VOCs in the beginning, its activity quickly decreases. However, Pd/ZSM-5 and Pd/SBA-15 show a good stability, without activity loss in a 72 h test. Our research reveals that higher acidity can facilitate catalytic reaction and gives rise to higher activity, but it also enhances coke formation and decreases the catalyst stability. In addition, the porosity of supports has a certain impact on catalytic oxidation of VOCs. 1. Introduction Volatile organic compounds (VOCs) are an important class of air pollutants, which emit from many industrial processes and transport vehicles, and cause many environmental problems. Many techniques have been developed to eliminate VOCs in the last decades. Incineration is a convenient approach to convert VOCs into carbon dioxide and water. However, it wastes a large amount of energy, involves a high temperature operation, and generates lots of toxic byproducts. In comparison with the conventional thermal incineration, catalytic oxidation can be operated at a much lower temperature, and the selectivity of catalytic oxidation could be controlled. Therefore, catalytic oxidation is an energy-saving and high-efficiency technique to eliminate VOCs.1 It is well-known that the catalytic performance is the key factor determining the effectiveness of this technique, and thus much attention has been paid to develop catalytic materials and improve their catalytic performance. Porous materials with a high surface area and a large pore diameter are often used as the catalyst support to disperse active phase with more active sites. Moreover, the porous structure allows the VOC molecules to reach the inner active sites to be readily oxidized. It is well-known that mesoporous materials, such as MCM-41 and SBA-15, have very high surface areas and uniform pore diameter (2.0-10.0 nm), which are good for uniformly loading the catalytic active component and the consequent catalytic reactions. In particular, SBA-15 has a larger pore diameter, thick pore wall, and tunable pore size, which is more suitable to deal with bulky VOC molecules. On the other hand, microporous materials, such as Beta and ZSM-5, are intensively investigated as supports for many important industrial catalytic processes, including catalytic oxidation of VOCs.2,3 Zeolites belong to the well-known family of crystalline (alumino) silicates, with a good combination of properties like high * To whom correspondence should be addressed. Tel.: +86 10 62849194 (Z.H.); +61 7 33463809 (Z.P.X.). Fax: +86 10 62923564 (Z.H.); +61 7 33463973 (Z.P.X.). E-mail: [email protected] (Z.H.); [email protected] (Z.P.X.). † Chinese Academy of Sciences. ‡ The University of Queensland.

surface area, well-defined microporosity, high hydrothermal stability, intrinsic acidity, and the ability to confine active metal species in the pores. Normally, pure support materials are nearly inert to the catalytic oxidation reactions at a low temperature due to the absence of active sites, and therefore many efforts have been focused on introducing active sites into mesoporous and microporous supports in the past decades. Various transition metals have been incorporated into the framework or dispersed onto the surface of siliceous materials as metal or metal oxides.4,5 In particular, supported noble metals or metal oxides are the most widely used catalysts in catalytic oxidation of VOCs6-8 through detailing the relationship between preparation conditions and catalytic activities of catalysts. However, the systematic studies on influences of molecular sieves with various frameworks and characteristics (such as the textural properties and acidities) on the catalytic activities have seldom been reported yet. In this Article, we reported the effect of five micro-/ mesoporous supports on catalytic oxidation of benzene, toluene, and ethyl acetate (typical VOCs) over Pd catalysts. The primary purpose of this contribution is to investigate the feasibility and distinction of catalytic elimination of VOCs over various molecular sieves, as these porous supports possess different strength and number of acid sites and have various Pd loading properties, which determine the catalytic performance. The prepared catalysts are characterized by XRD, ICP-OES, O2TPO, NH3-TPD, N2 adsoption/desorption, H2 chemisorption, and TEM techniques, and their catalytic activities were investigated in a continuous-flow fixed-bed reactor. 2. Experimental Section 2.1. Catalyst Preparation. In a typical synthesis of SBA15, 1 g of nonionic triblock copolymer surfactant pluronic P123 (EO20PO70EO20, Sigma-Aldrich) was dissolved in 35 mL of HCl aqueous solution at pH 2.0, followed by adding 2.3 mL of tetraethyl orthosilicate (TEOS) under stirring. After being stirred at 35 °C for 24 h, the mixture was transferred into an autoclave

10.1021/ie900412c CCC: $40.75  2009 American Chemical Society Published on Web 07/02/2009

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and aged at 100 °C for 48 h. The solid product was filtered and thoroughly washed with deionized water and dried at 100 °C for 24 h. After calcination in a muffle furnace at 550 °C for 6 h at a heating rate of 5 °C/min to remove the surfactant, the mesoporous SBA-15 was finally obtained. In a typical synthesis of MCM-41, 0.54 g of NaOH was dissolved in 30 mL of deionezed water, followed by adding 6 mL of TEOS, 6.36 g of CTAB, and 0.297 g of NH4F, and vigorously stirring for 1 h. The mixture was then transferred into an autoclave and aged at 110 °C for 10 h. The resultant solid was filtered, thoroughly washed, and dried at 80 °C for 24 h. The sample was finally obtained after calcination at 550 °C for 6 h. As for the synthesis of MCM-48, 3.73 g of CTAB was dissolved in 25 mL of water, followed by adding 0.48 g of NaOH and 5.26 g of TEOS, and vigorously stirring for 30 min. The mixture was then transferred into an autoclave and aged at 110 °C for 72 h. The resultant material was filtered, thoroughly washed and dried at 80 °C for 24 h, followed by calcining at 550 °C for 6 h to obtain the final product MCM-48. ZSM-5 (Si/Al ) 25) and Beta (Si/Al ) 16) zeolites were bought from Tianjin Chemical Plant. Pd catalysts with a Pd loading of 0.3 wt % were prepared by impregnating these porous supports with a PdCl2 aqueous solution and drying at 100 °C overnight, followed by calcining at 500 °C for 4 h and being reduced in a pure H2 stream (30 mL/min) at 480 °C for 2 h. 2.2. Catalyst Characterization. Small-angle XRD patterns were recorded on a SIEMENS D5005D powder diffraction system using Cu KR radiation (λ ) 0.15418 nm) in the 2θ range of 0.7-10° with a scanning rate of 0.5°/min. Wide-angle XRD patterns were measured on a Rigaku powder diffractometer (D/ MAX-RB) using Cu KR radiation in the 2θ range of 5-50° with a scanning rate of 4°/min. The Pd contents of as-synthesized catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), which was performed on an OPTIMA 2000. Before measurement, the samples were dissolved in a 1:2 mixture of aqua regia and 0.5 M (NH4)HF2 at 60 °C for 24 h. The Pd loadings were obtained from the emission intensities by means of a calibration curve. The N2 adsorption/desorption isotherms of catalysts at 77 K were collected on a gas sorption analyzer NOVA1200. All of the samples were degassed under vacuum at 300 °C for 3 h before the measurement. The total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/ P0) of ca. 0.99. The specific surface area (SBET) was calculated with the Brunauer-Emmett-Teller (BET) method (the micropore surface area and micropore volume were estimated by a t-plot method), and the pore size distribution (PSD) was derived from the desorption branch of the N2 isotherm using the Barrett-Joyner-Halenda (BJH) method. TEM images were collected on a Hitachi H-7500 microscope operating at an accelerating voltage of 80 kV. All samples were ground, dispersed in ethanol, and deposited on the copper grids prior to observation. Catalyst acidity was evaluated with temperature-programmed desorption of ammonia (NH3-TPD) in a Micromeritics chemisorb 2720. Typically, 0.1 g of catalyst was pretreated in a helium flow (50 mL/min) at 300 °C for 1 h and then cooled to room temperature prior to adsorption of NH3. After being saturated with NH3, the catalyst was flushed with He (50 mL/min) for 1 h at room temperature to remove the physisorbed NH3 from

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the catalyst surface. Next, the desorption profile was recorded from 150 to 550 °C at a heating rate of 10 °C/min. Coke deposition on the catalysts was analyzed by O2 temperature-programmed oxidation (O2-TPO) in a Micromeritics chemisorb 2720. In each experiment, the catalyst (0.1 g) collected from the VOC oxidation test was purged in a He flow (50 mL/min) at 300 °C for 1 h. After the catalyst was cooled to 25 °C, an O2/He mixture (5%/95% in v/v) was introduced at 50 mL/min, the temperature was ramped up to 650 at 10 °C/ min, and the TCD signal of the effluent gas mixture was monitored. The palladium dispersion on the support was assessed by chemisorption of H2 at 25 °C, that is, the molar ratio of chemisorbed hydrogen atoms to the total palladium atoms (H/ Pd), assuming that each exposed Pd atom adsorbs one hydrogen atom.9 The mean Pd crystallite size was further estimated from the equation: d (nm) ) 112/(percentage of Pd exposed),6 assuming that the Pd crystallites were spherical with a surface atom density of 1.27 × 109 atoms/m2. 2.3. Catalytic Oxidation Activities. All of the catalytic oxidation experiments were performed in a conventional fixedbed continuous-flow reactor under atmospheric pressure. It consists of a 6 mm i.d. stainless steel tube located inside an electrical furnace. The temperature in the catalyst bed and the tubular electric furnace was monitored automatically with E-type thermocouples, respectively. The vapor of benzene, toluene, or ethyl acetate was generated by passing air at a certain flow rate through the generator. In each test, 0.3 g of catalyst (40-60 mesh) was placed in the middle of the reactor. The O2 feed concentration was kept at about 21% (volume ratio); therefore, in all experiments, the flow rate of O2 was well above the stoichimetric amount required for complete oxidation of VOCs, as the molar ratio of O2/VOC is more than 100. The gas hourly space velocity (GHSV) for all tests was kept at 26 000 h-1. An online gas chromatograph equipped with an FID was used to analyze VOCs concentrations in the feed and effluent streams. Before each test, the catalyst bed temperature was raised to 130 °C with the feed stream passing (no oxidation at this temperature) and stabilized for 30 min, and then was raised at a heating rate of 5 °C/min to the next temperature point and stabilized for 20 min prior to collecting the GC data for the effluent stream. A blank test was similarly conducted only with quartz beads in the reactor, revealing that the oxidation of three VOCs was negligible at a temperature less than 300 °C. Long-term stability of catalysts was tested with 1500 ppm of benzene in the feed stream. The operation temperature and GHSV were fixed at 270 °C and 26 000 h-1, respectively, and the oxidation conversion of benzene was evaluated for 72 h. 3. Results and Discussion 3.1. Structural and Textural Properties of Catalysts. Figure 1A shows the small-angle XRD patterns of Pd/SBA-15, Pd/MCM-48, and Pd/MCM-41. Pd/SBA-15 displays a wellresolved pattern with a sharp diffraction peak at 2θ ) 0.8° and two small peaks at 1.4° and 1.6°, well matching with that reported for pure SBA-1510 and indexed as (100), (110), and (200) plane diffractions of the two-dimensional hexagonal mesostructure with a space group of p6mm symmetry. The diffraction pattern of Pd/MCM-48 is also similar to pure MCM48,11 with the basal diffraction peaks at 2θ ) 2.7° and 3.1°, corresponding to (211) and (220) diffractions of the Ia3d cubic structure.11 Pd/MCM-41 has the diffraction peaks at 2θ ) 2.3°, 4.5°, and 6.0°, due to (100), (110), and (210) reflections,

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Figure 1. (A) Small-angle X-ray diffraction patterns of catalysts: (a) Pd/ SBA-15, (b) Pd/MCM-48, and (c) Pd/MCM-41. (B) Wide-angle X-ray diffraction patterns of catalysts: (a) Pd/Beta, (b) Pd/ZSM-5.

respectively,12 showing that Pd/MCM-41 has the two-dimensional mesostructure like the pure MCM-41. Figure 1B shows the wide-angle XRD patterns of Pd/SBA-15 and Pd/Beta, evidencing the typical high crystallinity of zeolite Beta and ZSM-5.13,14 Therefore, the loading of Pd has negligible effect on the original porous structure of the support. Figure 2 shows the N2 adsorption/desorption isotherms and pore diameter distributions of the catalysts, and Table 1 lists the surface area (SBET), mean pore diameters (Dp), total pore

Figure 2. N2 adsorption/desorption isotherms (A) and pore size distribution (B) of various Pd catalysts: (a) Pd/SBA-15, (b) Pd/SBA-15-used, (c) Pd/ MCM-48, (d) Pd/MCM-48-used, (e) Pd/MCM-41, and (f) Pd/MCM-41used.

volume (Dv), unit cell parameters (a0), and pore wall thickness (W) of catalysts. These mesoporous samples exhibit the type IV adsorption/desorption isotherms with H1-type hysteresis loop, which are typical characteristics of mesoporous materials according to the IUPAC classification.12 As listed in Table 1, the freshly made Pd/SBA-15, Pd/MCM-41, and Pd/MCM-48 have a SBET of 662, 1031, and 1035 m2/g, and a Dv of 1.044, 0.727, and 0.810 cm3/g, with the pore size centered at 6.51, 2.23, and 2.43 nm (Figure 3B), respectively. To the contrary,

Ind. Eng. Chem. Res., Vol. 48, No. 15, 2009 Table 1. Physicochemical Properties of Catalysts sample Pd/SBA-15

status SBETa (m2/g) Dvb (cm3/g) Dpc (nm)

fresh tested Pd/MCM-41 fresh tested Pd/MCM-48 fresh tested Pd/ZSM-5 fresh tested Pd/Beta fresh tested

662 601 1031 283 1035 908 348 318 413 67

1.044 0.996 0.727 0.155 0.810 0.747 0.208 0.192 0.280 0.095

6.51 6.15 2.23 2.43 2.21

a0d (nm)

Wf (nm)

10.31

3.80

4.48

2.25

e

8.26

5.83

BET specific surface area. b Total pore volume estimated at P/P0 ) 0.99. c BJH pore diameter calculated from the desorption branch. d a0 calculated from d100, a0 ) 2 × d100/3. e For Pd/MCM-48, a0 calculated from d211, a0 ) d211 × 6. f Wall thickness ) a0 - Dp. a

the microporous Pd/Beta and Pd/ZSM-5 show a SBET of 423 and 348 m2/g, and a Dv of 0.280 and 0.208 cm3/g, respectively. Figure 3 shows the TEM images of SBA-15, Pd/SBA-15, and Pd/Beta. Figure 4 shows the Pd particle size distributions over Pd/SBA-15 and Pd/Beta. We can see that SBA-15 has welldeveloped parallel pore channels with a uniform diameter of ca. 7.2 nm (Figure 3A), which is well retained in Pd/SBA-15 after loading Pd (Figure 3B and C). We also note that the Pd particles are well dispersed over SBA-15 and Beta (Figure 3B-D), with the average particle diameters of Pd particles over Beta and SBA-15 being ca. 5.6 and 10.5 nm (Figure 4), respectively. The H/Pd molar ratio and the calculated average palladium particle size of these catalysts are listed in Table 2. In general, the H/Pd ratio of mesoporous materials supported catalysts appears to be higher than that supported on microporous materials. This result probably suggests that larger specific surface area and bigger pore size are good for the palladium dispersion on the support. The estimated mean crystallite size of Pd particles is smaller than that from the TEM images (Figure 3), probably because many Pd particles are too small to be observed in TEM, so that only larger visible Pd particles can be counted in TEM images. 3.2. Catalyst Acidity. The NH3-TPD was performed to evaluate the acid strength and acid sites quantities of the catalysts. Figure 5 shows that Pd/Beta and Pd/ZSM-5 have two distinct peaks in the range of 150-550 °C in the NH3-TPD profiles, which means that there are two types of acid sites. Pd/Beta seems to have the strongest acidity and the most acid sites as the two peaks are located at the highest temperatures (243 and 410 °C) with the largest NH3 desorption amount (1.51 mmol/g). Next to Pd/Beta, Pd/ZSM-5 also possesses a very strong acidity,15 as the NH3 desorption amount can reach up to 1.11 mmol/g (Table 2). In sharp contrast, Pd/MCM-41 and Pd/ SBA-15 only have a flat peak between 250 and 400 °C, indicating that the weak acid site is missed in these two catalysts (Figure 5). The strength and total numbers of acid sites have the following order: Pd/Beta > Pd/ZSM-5 > Pd/MCM-48 > Pd/ MCM-41 > Pd/SBA-15 (Table 2). 3.3. Catalytic Oxidation of VOCs. Catalytic oxidation of benzene (1440 ppm), toluene (650 ppm), and ethyl acetate (1100 ppm) was carried out over these supported Pd catalysts. Their conversion profiles in the first test are illustrated in Figure 6. In general, the oxidation of three organic compounds starts at 160 °C, and complete conversion (oxidation) occurs in the vicinity of 300 °C except for the cases using Pd/MCM-41 and Pd/MCM-48. Figure 6 also indicates that it is the easiest to completely oxidize ethyl acetate but it is the most difficult to oxidize toluene, which is also supported by the temperatures

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for 50% and 90% oxidation conversion (Table 3). In terms of the catalytic activity, Pd/Beta seems to cause the highest oxidation of three compounds at each temperature point, followed by the order of Pd/ZSM-5, Pd/SBA-15, Pd/MCM-48, and Pd/MCM-41. For example, at 250 °C, Pd/Beta leads to 100% oxidation of three compounds, but Pd/ZSM-5 causes oxidation of 86% benzene, 62% toluene, and 96% ethyl acetate, and Pd/SBA-15 brings about oxidation of 77% benzene, 40% toluene, and 82% ethyl acetate. Relatively, Pd/SBA-15 exhibits a better catalytic performance in the oxidation of VOCs than Pd/MCM-48 and Pd/MCM-41 (Figure 5). This could be related to the active redox sites are relatively better dispersed on SBA-15 (1.66 nm), which can obviously promote the catalytic oxidation activity. The other reason is the pore geometry of catalyst. Pd/SBA-15 has the largest pore diameter and the biggest pore volume (Table 1), so that the VOC molecules can readily reach the redox sites (Pd particles) and the acid sites located in the mesopores. Note that Pd/MCM-41 shows the lowest catalytic activity, and complete oxidation of benzene cannot be achieved even at 410 °C, which could be related to its relatively poor Pd particles dispersion on the support (2.51 nm) and low hydrothermal stability16,17 (Table 1), discussed shortly in the next section. The higher activity of Pd/Beta and Pd/ZSM-5 may be attributed to its strong and large quantity of acid sites, as the catalytic activity in oxidation of VOCs18 is probably related to the acid strength and acid sites number, which are good for the active sites dispersion on the supports and CO2 desorption from the catalysts during catalytic reactions. As is known, the twostep redox model is usually proposed for oxidation of hydrocarbons (HCs) over supported Pd catalysts.19 This model assumes reaction occurs when reactant molecule interacts with an oxygen-rich portion of the catalyst, and a portion of the catalysts surface is alternately reduced (Pd0) and oxidized (PdOx). In this work, there is no PdO species (only Pd0) over the fresh samples as all of the fresh samples are reduced in a pure H2 stream at 480 °C, and when O2 is present in the reactant feed mixture, a portion of Pd0 gets oxidized to PdO, and subsequently reduced by the HCs. As a consequence, both the Pd0 and the PdO are responsible for the oxidation reaction. The two-step redox model is as follows: Cat-O + HC f Cat + CO2

(1)

That is, a portion of oxidized catalyst is reduced by the hydrocarbon (HC). In this step, HC is oxidized to CO2 on the catalyst surface, and then CO2 is desorbed from the surface to the gas stream. 2Cat + O2 f 2Cat-O

(2)

In the second step, the catalyst redox center is oxidized by the stream O2 to recover the active oxidized centers through adsorption and dissociation of O2 on the Pd surface. Acid sites are proposed to promote desorption of CO2 from the catalyst surface, so that reaction 1 could be directly facilitated, and simultaneously more reduced catalytic centers are released and reaction 2 is sped up. 3.4. Stability and Deactivation of Catalysts. The stability of three good catalysts (Pd/SBA-15, Pd/ZSM-5, and Pd/Beta) was tested for 72 h under conditions of 1500 ppm of benzene in the feed gas, reaction temperature of 270 °C, and GHSV of 26 000 h-1. The evolution of benzene conversion with timeon-stream for 72 h is shown in Figure 7. During the 72 h test, the conversion of benzene over Pd/SBA-15 is well sustained at

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Figure 3. TEM images of catalysts: (a) pure SBA-15, (b,c) Pd/SBA-15, and (d) Pd/Beta.

Figure 4. Pd particle size distribution of catalyst (A) Pd/SBA-15 and (B) Pd/Beta. Table 2. Characteristic Data of As-Synthesized Catalysts sample Pd/SBA-15 Pd/MCM-41 Pd/MCM-48 Pd/Beta Pd/ZSM-5

dc Sqd Wqe Tf Pda (wt %) H/Pdb (nm) (mmol/g) (mmol/g) (mmol/g) 0.28 0.27 0.28 0.28 0.27

0.673 0.446 0.511 0.379 0.388

1.66 2.51 2.19 2.95 2.88

0.02 0.03 0.09 1.04 0.78

0.20 0.47 0.33

0.02 0.03 0.29 1.51 1.11

a Actual Pd contents obtained by the ICP analysis. b Molar ratio of adsorbed hydrogen atoms to the total palladium atoms. c Calculated diameters of the palladium crystallites based on the dispersion of Pd. d Amount of desorbed NH3 on strong acid sites. e Amount of desorbed NH3 on weak acid sites. f Total amount of desorbed NH3.

90.5% ((1.5%) without noticeable activity loss. The conversion over Pd/ZSM-5 seems to be well maintained, with its activity fluctuated to some extent at 91.5% ((4%). In sharp contrast, the activity of Pd/Beta is gradually reduced from 100% at the beginning to 38% after the 72 h test.

Figure 5. The NH3-TPD profiles of different samples: (a) Pd/Beta, (b) Pd/ ZSM-5, (c) Pd/MCM-48, (d) Pd/MCM-41, and (e) Pd/SBA-15.

To understand the deactivation phenomena, the tested catalysts were further characterized with the O2-TPO technique, as porous material-supported materials in catalytic oxidation of VOCs probably form the coke,20 which can lead to catalyst

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Figure 7. Stability tests for benzene oxidation with time-on-stream over different catalysts: (a) Pd/SBA-15, (b) Pd/ZSM-5, and (c) Pd/Beta.

Figure 8. The O2-TPO profiles of tested catalysts: (a) Pd/Beta, (b) Pd/ ZSM-5, (c) Pd/MCM-48, (d) Pd/MCM-41, and (e) Pd/SBA-15.

Figure 6. Conversion profiles of different VOCs oxidation over Pd catalysts loaded on various porous supports: (a) benzene, (b) ethyl acetate, and (c) toluene. Table 3. Catalytic Activity Data and Amount of Coke of Catalysts T50a (°C)

sample Pd/Beta Pd/ZSM-5 Pd/SBA-15 Pd/MCM-41 Pd/MCM-48

T90a (°C)

coke (mg C/g ethyl ethyl cat) benzene toluene acetate benzene toluene acetate 104.6 9.8 0.8 3.7

206 226 231 291 268

226 238 261

194 197 230

267

239

225 264 268 367 332

245 272 292

213 226 256

301

299

a

Temperatures at which 50% and 90% conversion of benzene, toluene, and ethyl acetate, respectively.

deactivation. Figure 8 shows the oxidation profile of the coke deposited on the used catalysts. Pd/Beta exhibits a strongest peak at 559 °C, meaning that the amount of coke deposited (104.6 mg C/g cat) over Pd/Beta is much more than other catalysts. The other extreme case is that Pd/SBA-15 does not show any TCD signal (Figure 8), indicating that there was limited coke deposited on this catalyst. Based on the peak area

of TCD signal, the coke amount deposited on the catalyst is decreasing in the following order: Pd/Beta > Pd/ZSM-5 > Pd/ MCM-48 > Pd/MCM-41 > Pd/SBA-15 (Figure 8). This order is the same as that for the strength and number of acid sites estimated from the NH3-TPD (Figure 5 and Table 2). As reported elsewhere, coking is closely related to the strength and number of acid sites, especially the strong acid sites. During catalytic reactions, some reactive intermediates are oxidized to CO2 and H2O, while some others are converted to coke and deposit on the active sites. Moreover, if the deposition takes place on the active sites near the channel mouth that is more readily accessible, coke is more easily formed and blocks the inner pores.21 Both the coke deposition and the pore blocking reduce the catalytic activity, and thus prohibit catalytic oxidation of VOCs. The coke formation has also been reflected by the change of the catalyst textural properties. As shown in Table 1, both SBET and DV of used Pd/Beta are significantly reduced to only 20-30% of the original values. It is true that Pd/Beta has the strongest and the most acid sites and shows the highest activity in the first few hours; however, due to the severe coke formation, its long-term activity is gradually lost, as shown in Figure 7. Pd/MCM-41 possesses little acid sites; however, its SBET and DV show 72.5% and 78.6% reduction, respectively, which is due to its low hydrothermal stability,17,18 as the mean pore diameter of the spent Pd/MCM-41 sample is less than 1 nm (Figure 2B), indicating the collapse of its pore structure. The other three catalysts show only 10% reduction in SBET and DV, indicating that the coke formation is not so severe, which may be the reason that Pd/SBA-15 and Pd/ZSM-5 exhibit the longterm stability in the catalytic performance (Figure 7). It is understood that Pd/SBA-15 has weaker and less acid sites, so that the coke formation is limited. However, Pd/ZSM-5 shows relatively strong and more acid sites (Table 2), but the coke

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formation is also restricted (9.8 mg C/g cat) in comparison with Pd/Beta. The low coking tendency on Pd/ZSM-5 may be due to the steric constraints exerted by its porous network on the formation of the large intermediates in coking.22 4. Conclusion The performances of catalytic oxidation of benzene, toluene, and ethyl acetate over various micro-/mesoporous materialsupported catalysts were investigated. A larger SBET and a bigger pore diameter are favorable to the active redox component dispersion. The catalytic oxidation activity of freshly made catalysts is as follows: Pd/Beta > Pd/ZSM-5 > Pd/SBA-15 > Pd/MCM-48 > Pd/MCM-41. For microporous catalysts, the strength and quantity of acid sites probably play an important role in their catalytic oxidation of VOCs. As for mesoporous catalysts, it seems that their textural properties are the most important factors. As a whole, the acidity has improved catalytic oxidation of these VOCs, but also induced quick coking and deactivation. As a result of trading off of the acidity strength and site number and suitable porosity, Pd/ZSM-5 and Pd/SBA15 show a good long-term stability in catalytic oxidation of these VOCs, appearing as the promising catalyst for the practical application in VOCs oxidation with a lower cost. Acknowledgment The National Basic Research Program of China (No. 2004CB719500), the National High Technology Research and Development Program of China (No. 2006AA06A310), and the National Science Fund for Distinguished Young Scholars (No. 20725723) are gratefully acknowledged. Literature Cited (1) Li, J. J.; Xu, X. Y.; Jiang, Z.; Hao, Z. P.; Hu, C. Nanoporous SilicaSupported Nanometric Palladium: Synthesis, Characterization, and Catalytic Deep Oxidation of Benzene. EnViron. Sci. Technol. 2005, 39, 1319. (2) Guo, J. J.; Lou, H.; Zhao, H.; Zheng, L. H.; Zheng, X. M. Dehydrogenation and Aromatization of Propane over Rhenium-Modified HZSM-5 Catalyst. J. Mol. Catal. A: Chem. 2005, 239, 222. (3) Lucas, A. D.; Valverde, J. L.; Sa´nchez, P.; Dorado, F.; Ramos, M. Hydroisomerization of n-Octane over Platinum Catalysts with or without Binder. Appl. Catal., A 2005, 282, 15. (4) Zhang, W. H.; Lu, J. Q.; Han, B.; Li, M. J.; Xiu, J. H.; Ying, P. L.; Li, C. Direct Synthesis and Characterization of Titanium-Substituted Mesoporous Molecular Sieve SBA-15. Chem. Mater. 2002, 14, 3413. (5) Newalkar, B. L.; Olanrewaju, J.; Komarneni, S. Direct Synthesis of Titanium-Substituted Mesoporous SBA-15 Molecular Sieve under MicrowaveHydrothermal Conditions. Chem. Mater. 2001, 13, 552. (6) Li, J. J.; Jiang, Z.; Hao, Z. P.; Xu, X. Y.; Zhuang, Y. H. Pillared Laponite Clays-Supported Palladium Catalysts for the Complete Oxidation of Benzene. J. Mol. Catal. A: Chem. 2005, 225, 173.

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ReceiVed for reView March 13, 2009 ReVised manuscript receiVed June 14, 2009 Accepted June 17, 2009 IE900412C